In a multi-phase DC-DC converter which supplies an electric current to a load by connecting a plurality of power stages each built up by a switch and an inductor, if there is dispersion of elements employed for the respective power stages, the currents supplied from the individual power stages do not become equal to each other.
At this time, a large current concentrates on a specified power stage, and such a possibility exists that the elements like the switch, the inductor, etc. in the power stage might be broken down. Further, even when not reaching the breakdown, there exists also a possibility of causing a decrease in inductance value of the inductor on which the current concentrate due to a DC superimposed characteristic of the inductor.
In order to eliminate influence thereof, a known feedback control method is a method of evenly balancing the respective currents in a way that observes the currents supplied to the load from the respective power stages.
On the other hand, it is known that duty ratios of the individual power stages may be evenly controlled in terms of conversion efficiency. At this time, the current supplied to the load depends on a parasitic resistance existing in a current path, and a current value becomes smaller as a resistance value gets larger. Accordingly, the currents, which are output by the power stages when the efficiency is maximized, are not equalized.
Thus, the conventional multi-phase DC-DC converter has such a problem that the conversion efficiency decreases when equalizing the currents output by the power stages in order to protect the elements.